The present invention relates to novel non-slumping, pumpable castables containing calcium-aluminate cement, and to the method of applying the same without the utilization of forms.
The use of wet gunning mixes and systems and shotcreting systems are old and well-known procedures. For example, shotcreting of Portland cement based concretes comprises forming a mix containing sand, gravel, and a Portland cement and tempering with water to a pumpable consistency. This concrete is then placed through a swing valve concrete pump and conducted to a nozzle where a cement set accelerator and air are added to blow the material onto a surface where it stiffens rapidly enough so as not to slump off the surface to which it is applied, such as a wall. This stiffening reaction is a result of the chemical accelerator reacting with Portland cement to rapidly begin a setting action, which quickly raises the viscosity of such concrete to a level high enough to prevent slumping. Approximately 3 to 6 wt. % accelerator, for each 100 wt. % of dry mix, is required to reach the desired stiffness. Accelerators cause hydration reactions that stiffen the cement.
However, while this is suitable for many uses, refractory concretes are not satisfactorily used with Portland cements as a binder, because they cannot withstand the high temperatures and corrosive environments in which refractory materials are placed. Thus, a large majority of refractory concretes, also known as refractory castables, contain calcium-aluminate cement instead of Portland cements. Calcium-aluminate cements have a much higher melting temperature, much higher refractoriness, and higher corrosion-resistance to the environment seen in high temperature refractory applications. Such high temperature refractory applications can range from boiler ash hoppers to steel ladles.
However, calcium-aluminate cements have very different hydration reactions compared to Portland cement, so that additives used, for example, in the shotcreting of Portland cement concrete do not work well in refractory castables with like amounts of calcium-aluminate cement. As a consequence, at the present time there is not commercially available any refractory shotcrete composition. Moreover, while calcium-aluminate cement is vastly superior to Portland cement in its ability to withstand, as noted, high temperatures and corrosive environments, it is still not able to withstand such environments as well as typical refractory grains, such as high purity alumina, andalusite, calcined alumina-silica or amorphous silica grains. The calcium-aluminate cement, therefore, remains the weak link in the refractory castable.
By virtue of this fact, refractory companies have developed castable systems that use very low amounts of calcium-aluminate cement. This is possible by using chemicals that reduce the amount of water needed to bring the castable to a consistency that it can be installed, the use of sub-micron particles as flow aids, and the careful control of the particle size distribution to further lower the amount of water needed to achieve suitable pumpable consistency and to increase the amount of grain to grain contact to keep the strengths high. These low calcium-aluminate cement castables have found uses in a variety of applications over the past decades where previously only pressed brick could be utilized. Such applications vary from steel ladles to aluminum melting furnaces. The savings realized by the use of casting compared to the laying and mortaring of brick are quite high in terms of labor costs and time. However, all of the cost savings cannot be realized, because forms are needed to place these castables into a suitable lining and such preparation, installation, and removal of such forms is in itself a costly procedure, not only in terms of time but also in terms of labor to install and remove the same after the refractory has hardened.
In an effort to overcome these problems, gunite materials have been utilized and are placed into position by pneumatically conveying the dry material through a hose to a nozzle where water is added. This method, however, gives inferior properties compared to casting and has the added disadvantage of producing rebounds and a significant amount of dust. Rebounds consist of the material that does not stick to the target wall or ceiling and bounces onto the floor. Efforts to reduce dust and to eliminate rebounds, such as by the addition of granular or powdered calcium chloride to a relatively high calcium-aluminate cement systems still do not achieve suitable castables, particularly in terms of the physical properties needed.
Another procedure to overcome the problems with guniting has been the addition of accelerators, such as aluminum sulfate or gypsum, at the nozzle of a conventional guniting system in order to accelerate the cure of the cement and to prevent slumping. Such systems heretofore have been all lightweight mixes, which initially reduces their tendency to slump, and while useful in some applications, it does eliminate a great deal of the rebounding and dust, and it is not suitable for refractory vessels such as steel or iron ladles, and the like, which are exposed to highly destructive environments and require dense strong products which cannot be made utilizing such accelerators. In like manner, other techniques have been utilized in an effort to eliminate the cumbersome equipment required by many of the procedures, some of which require multiple components and no satisfactory procedure or composition has yet been found to be able to install a low cement, low moisture non-slumping calcium-aluminate cement-containing pumpable castable.